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Vacuum tube rejuvination
Original poster: Brett Miller <brmtesla2@xxxxxxxxx>
Thank you for that excellent description of thorium
oxide depletion in tube filaments. I will also read
the articles you have linked. This is exactly what I
wanted to know.
--- Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: "huil888" <huil888@xxxxxxxxxxxx>
> In a vacuum-tube with a directly-heated
> thoriated-tungsten filament,
> the actual source of electrons is thorium oxide,
> which is alloyed
> with the tungsten filament. The thorium is brought
> to the surface of
> the filament during special processing when the tube
> is built. As
> this surface layer of thorium is slowly depleted
> during tube
> operation, electron emission gradually drops. Very
> slight increases
> in temperature greatly increase the rate of thorium
> depletion, which
> is why tube manufacturer's specify VERY close
> control of filament
> voltage (= filament temperature).
> With a tube that is already showing reduced
> emission, a way to
> temporarily increase emission is to increase
> filament temperature.
> However, this will relatively quickly deplete the
> remaining thorium.
> Nevertheless, given the total run time of a typical
> VTTC, this may
> yield acceptable results.
> There are a number of papers available on the web on
> increasing the
> filament emission of "tired" vacuum tubes, and also
> guidelines on
> first firing-up high power vacuum tubes that have
> sat in storage for
> many years. Search on "vacuum tube rejuvenation" or
> similar topics. A
> general recommendation seems to be to run the tube
> for some period of
> time with filament power applied, but with no B+
> applied. 24-48 hours
> of operation in this mode allegedly helps to de-gas
> the tube, and
> reduce the potential of internal arcing.
> Here is an article describing the "rejuvenation" of
> Here is a relatively recent paper describing
> low-emission problems
> with high power RF tubes used in the CERN nuclear
> accelerator, and how filament power monitoring and
> control led to
> greatly increased tube life.
> Scott Hanson
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